Since recently it is known that certain substances that may be present in the body can function as indicators for various pathological conditions in the body. Such substances are hereafter called indicator substances. Examples of indicator substances are glucose, lactate, pyruvate, glycerol, glutamate, and glutamine and heart specific enzymes. Pathological conditions that may be indicated or detected, or as well forecasted, include ischemia, hypoglycemia sepsis, cell membrane damage or lipolysis, vasospasms and metabolic disorders. By measuring indicator substances, pathological conditions may be detected before they lead to clinical signs. It may even be possible to detect processes or conditions that eventually may lead to a pathological condition. In many cases it would be advantageous to have the possibility to measure the concentration of indicator substances directly in a blood stream, or in tissue fluid. However, until now there have not existed any systems suitable for clinical use for measuring indicator substances. Systems known from the background art all have different drawbacks. Examples of common drawbacks in background art systems are that the measurement delay is extensive and that one has measured phenomena that are the result of a pathological condition, e.g. ischemia. This is clearly disadvantageous. With measurement delay is meant the time that passes from the moment that a sample is taken until the moment that a measurement value relating to this sample is obtained. In background art systems also measurement values can often only be obtained with relatively extended time periods, between each measurement value, e.g. if sample fluid is collected in micro vials. Faced with the aim or task to develop a reliable and accurate measuring system that can be used in monitoring the condition of a subject, e.g. a patient, in a critical condition or situation, the skilled person is faced with other problems and situations than those which previously have aroused.
From U.S. Pat. No. 5,078,135 there is known a measuring system where a drug is administrated to a rat and where a microdialysis probe is placed in the vein of the rat. Mass spectrometry is used to batchwise analyse the dialysate for obtaining pharmacokinetic data.
From US-A1-2004/0191848 there is known a system for measuring the concentration of glucose in tissue fluid. A microdialysis probe is used which is fed with a perfusate fluid already containing glucose. The concentration of glucose in the perfusate fluid is controlled using self-adaptive control.
It is an object to provide a measuring system that is improved with respect to the background art. A further object is to provide a system that is reliable and accurate to make the system suitable for clinical use with such suitable response times and the system is useful for on-line monitoring in critical care.